This invention relates in general to densitometers and, more particularly, to an improved method and apparatus for graphically displaying densitometer output. Densitometers are, of course, well known as devices which scan a sample and provide an output signal or graphical display indicative of the optical density of the scanned sample.
One well known use of the densitometer is to scan a sample of blood which has been prepared by the electrophoresis process. Electrophoresis of blood samples isolates various proteins in the blood, known as albumin, alpha-1 globulin, alpha-2 globulin, beta-globulin and gamma-globulin. The electrophoresis technique separates these proteins from each other and then the sample may be processed or scanned in a densitometer. Each of the proteins exhibits a different light absorption characteristic or pattern and the light absorption patterns are graphically displayed by the densitometer to indicate the presence and quantity of each protein.
In optical density analysis, the amount of light passing through the sample is an inverse logarithmic function of the optical density of the sample. Thus, if the optical density of the sample is doubled the transmitted light is reduced by a factor of ten. The light transmitted through the sample falls on a photo-responsive element which generates electrical signals having a current proportional to the amount of transmitted light. The current output of the photo-responsive element is, therefore, also a logarithmic function of the optical density which then is converted into analog or time varying signals directly proportional to the optical density pattern of the scanned sample. The analog signals drive a graphic display unit to provide a permanent curve or record of the optical density pattern. All this is well known.
The analog signals, when graphically displayed, exhibit a series of peaks and valleys. In the analysis of blood, the area under the optical density curve and bounded by the two adjacent valleys separated by one peak, is representative of the quantity of each protein in the sample. The important data is the relative percentage of each protein, and the selection of these boundaries, i.e., the precise locations of these valleys, is arbitrary and hence is the basic problem since errors will result in inaccurate analysis of the sample. The problem in not unique to evaluation of blood samples but is common to optical and magnetic density evaluations and, in fact, to all evaluations of analog data.
There are various prior art systems which have considered this problem and which provide a standardized graphical display of the densitometer output. For example, U.S. Pat. No. 3,706,877 issued Dec. 19, 1972 to George F. Clifford, Jr., et al, and U.S. Pat. No. 3,784,789 issued Jan. 8, 1974 to Vanden Broek, each disclose a system for analyzing the densitometer output. The densitometer output is graphically displayed as an analog signal or curve indicative of optical density and a second signal which is the integral of the optical density, i.e., the area under the optical density curve, and which may be either analog or numerical, or both.
There are three common techniques for determining the location of these boundaries or valleys. In a first technique, the densitometer includes circuitry to automatically detect the valleys between the peaks based upon changes in the slope of the curve, integrate the area under the curve between valleys and print out the integral in numerical form. Then, in order to determine the percent of each protein, the operator of the equipment has to add the printed values to obtain a denominator and then calculate each percent by dividing each printed value by the calculated denominator.
Not only is this time consuming but if, in fact, the computer system erroneously selected a particular boundary location, the results are useless to the physician evaluating the blood sample. Hence, the results of a computer print-out cannot conveniently be utilized for subsequent evaluation if the physican analyzing the blood sample disagrees with the particular boundaries selected by the computer.
The second type of system provides two graphic displays, the first being the optical density of the scanned sample and the second being the integral of the optical density pattern. These are plotted or traced on graph paper and at a later time, the physician can select the particular boundaries for each protein. Then the area under the curve is calculated by actually manually counting the number of squares under the curve between each pair of boundaries which is equivalent to the aforementioned printed values. Again the percentage of each protein is then calculated by the addition and division procedure explained previously. Again, however, this requires laborious manual counting as well as manual calculations.
Finally, the third type of system provides both of these techniques together, i.e., a numerical print-out and a curve, so that the boundaries may be manually selected if the physician is not satisfied with the automatically selected boundaries. However, this still does not eliminate the manual counting and addition-division procedure for obtaining percentages of each protein.